Selecting healthy fish for tropical aquariums

One of the most dangerous times for your fish tank is the introduction of new fish into the ecosystem, and learning to identify healthy fish for tropical aquariums is one of the most useful skills you can develop as an aquarist.

At Swell UK, we still hear about people getting it wrong from time to time, but we are happy to say we have a few tips to help you make getting a new fish as safe as possible.


Before we even discuss what to look out for with your new fish, it’s worth noting that the only completely safe way to add new fish is to quarantine them first. When you buy a fish from a stockist, whether that’s in person or ordered via the phone or internet, you have very little time to assess the health of the fish before adding it to your tank.

Keeping a quarantine tank operating is the safest way then. By placing your new fish into this tank for a week or so allows you the time to better access your fish, ensuring it doesn’t present any diseases for a good while before you add it to your tank, where it could otherwise infect other fish, potentially with fatal consequences.

Ensure your quarantine tank has a good water quality, and best matches the final aquarium environment you intend for your new fish.

What are the signs of a healthy or unhealthy fish?

Generally speaking, healthy fish have good colouring and are active. Fish benefit from thousands of years of evolution, meaning that for most species, swimming looks almost effortless, with great speed created with just a flick of the tail. Anything short of this kind of movement may indicate that they have problems with their muscles, nervous system, or their fins and tail. Be sure to check carefully for fin rot or problems with their gills.

Clouded eyes may be a sign of parasites of other diseases. Ensure that if the eyes are clouded, this is a feature of the species, and not an illness ready to infect your other fish.

Skin and Scales:
Missing scales, raised scales and trailing worms are all bad signs. There are a number of diseases that can cause these symptoms, including parasites. Either stay clear of these fish and ones that have come into contact with it, or quarantine and medicate for an extended period before adding to your tank (we recommend leaving them be unless you can invest the time, money and effort).

Doesn’t the fish look well fed and not hollow bodied? Overly skinny fish may be ill, have parasites and may not have long left.

This might sound overly cautious, but it can make a huge difference. If you spot ANY fish in the shop that have diseases, ensure that the same net isn’t being used in all tanks. This can easily spread diseases from tank to tank, meaning that while your fish seems symptomless, if might not be in a few days time.

Georgina Posted by on

Georgina is a member of the Swell UK marketing team and has been keeping tropical fish for a number of years now. Her favourite fish being the stunning, male Siamese Fighting Fish. She is also looking to expand her existing collection to include keeping saltwater fish as well. Her other pets include Bengal cat, Walter, and Labrador and Rottweiler cross, Presley.


  • Avatar Michelle Schlosser Posted 05/07/2017 at 1:25 am | Permalink | Reply

    We just lost a large Angel fish & Bala shark within 4 days of each other. Couldn’t see anything wrong with the angel but the Bala had cloudy eyes. The tank water tests perfect. There is a yellow lab, pleco &2 silver slats left in a 55 gal tank. Any idea what’s wrong?

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